Well if you knew the names of the models' mesh and texture files you could replace the SS ones with the original ME2 ones. I for one don't know the names of all the mesh and texture files so you'll have to find that out yourself. Once you do though all you need to do is do some text editing in the battle_models file in SS's data folder.
Here's an example:
Say you wanted to change the model of the Polish Knights to something else. You're going to have to change these lines in red depending on where the files are in your Stainless Steel's models folder.
The next step is to change those lines. Depending on where the file is, in the models folder the line will change accordingly.
In red are all the things I've changed for this unit. Now the unit should have it's new model ingame.
Note that the numbers above the mesh line has changed this is because the model I'm using only has one mesh. When you text edit only edit the number to the right for that line. It indicates how many meshes there are that you are using. Also note that the numbers to the far left for the texture lines have changed. This number counts how many characters are on the line so be careful when you edit lines in the battle_models file. Now the unit should have the model you wanted to use for the unit. I hope this helps and teaches you how to edit the battle_models file for your own personal use and enjoyment. Any questions?